Ive often wondered if temperature is the prime motivator for the max RPM specs. Maybe the gearbox gets too hot, the grease inside becomes too thin to adequately lubricate or perhaps components begin to expand slightly and cause binding in areas that require tight tolerances to be maintained?

If that’s the case I know where we can find a bunch of coolant…

can anyone comment on gearbox temp during hard operation? It would be really cool to put the eagle tree temp probe down there although wire length/ material used to extend the wire down the mast may affect accuracy. Not sure if eagle tree probes are thermistors or thermocouples (or maybe even RTD’s)

Rating gears is difficult and there are many aspects to it. Things that are affecting them and their bearings are: heat, load, temperature, RPM and if there are load spikes. To calculate the lifetime, you combine all these factors in formulas together with some variables from the manufacturer and get an estimate. Reducing any of those variables increases the lifetime.
3 things that shorten the lifetime considerably are shock loading (going from forward to reverse instantly), excessive heat (unhardens the teeth) and lack of lubrication (with lubricant, the teeth never actually touch).

These gearboxes are probably used by industry to run for several years. Since we are only using them for a few hours, they could very well survive at higher speeds.

20,000hrs is OK, 6,000rpm isn’t super, so i would say you could use a motor like the 6374 168kV outrunners from Hobbyking, 12S and aim for 6,000 rpm loaded. Make your esc limited to 6,000rpm and you have to keep torque under 6Nm (middle sized one 5:1) or you’ll reduce it’s life. the 5:1 reduction gives you a big prop, which gives you better efficiency, but you eat an 6% loss in the box… which at 2kW is going to be 120W, ouch… that’s why I don’t like gear boxes. But the torque is very low compared to the Neugart. 6Nm at the prop is 1.2Nm at the input and I’m not sure it’s enough torque to do anything? The Neugart is 40Nm at shaft Nominal vs 6Nm… am I reading it right? If I am I think the Neugart handles 7x more power.

You have to differ: SSS has max ~2Nm input @max 25kRPM, so after gearbox: 10Nm @5kRPM
The 6374 has about 6Nm input @max 7,5kRPM, so after gearbox: 30Nm @1,5kRPM.
Look for props with ~0,8kRPM - 1,5kRPM rating. e.g. a solas 14"x19" is rated for 3,25kRPM max efficiency. And I don’t think you want to go higher than 14" diameter…most ppl here don’t want to go beyond 7" diameter…